ASME PRESSURE VESSELS
The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America. For more information about our products, heavy plate & custom fabrication services or fabrication capabilities contact us today!
Arc Welding Processes
Arc-Welding Fundamentals
Arc welding is one of several fusion processes for joining metals. By the application of intense heat, metal at the joint between two parts is melted and caused to intermix – directly or, more commonly, with an intermediate molten filler metal. Upon cooling and solidification, a metallurgical bond results. Since the joining is by intermixture of the substance of one part with the substance of the other part, with or without an intermediate of like substance, the final weldment has the potential of exhibiting at the joint the same strength properties as the metal of the parts.
In arc-welding, the intense heat needed to melt metal is produced by an electric arc. The arc is formed between the work to be welded and an electrode that is manually or mechanically moved along the joint (or the work may be moved under a stationary electrode). The electrode may be a carbon or a tungsten rod, the sole purpose of which is to carry the current and sustain the electric arc between its tip and the workpiece. Or, it may be a specially prepared rod or wire that not only conducts the current and sustains the arc but also melts and supplies filler metal to the joint. If the electrode is a carbon or a tungsten rod and the joint required added filler metal for fill, that metal is supplied by a separately applied filler-metal rod or wire. Most welding in the manufacture of steel products where filler metal is required, however, is accomplished with the second type of electrodes – those that supply filler metal as well as providing the conductor for carrying electric current.
Basic Welding Circuit
The basic arc welding circuit is shown in Figure 1. An AC or DC power source is connected by a ground cable to the workpiece and by a “hot” cable to an electrode holder of the same type, which makes an electrical contact with the welding electrode. When the circuit is energized and the electrode tip touched to the grounded workpiece, and then withdrawn and held close to the spot of contact, an arc is created across the gap. The arc produces a temperature of about 6500oF (3600oC) at the tip of the electrode, a temperature more than adequate for melting most metals. The heat produced melts the base metal in the vicinity of the arc and any filler metal supplied by the electrode or by a separately introduced rod or wire. A common pool of molten metal is produced, called a “crater”. This crater solidifies behind the electrode as it is moved along the joint being welded. The result is a fusion bond and the metallurgical unification of the workpieces.
Arc Shielding
Use of the heat of an electric arc to join metals, however, requires more than the moving of the electrode in respect to the weld joint. Metals at high temperatures react chemically with the main constituents of air – oxygen and nitrogen. Should the metal in the molten pool come in contact with air, oxides and nitrides would be formed, which upon solidification of the molten pool would destroy the strength properties of the weld joint. For this reason, the various arc-welding processes provide some means for covering the arc and the molten pool with a protective shield of gas, vapor or slag. This is referred to as arc shielding, and such shielding may be accomplished by various techniques, such as the vapor-generating covering on filler-metal-type electrodes, the covering of the arc and the molten pool with a separately applied inert gas or a granular flux, or the use of materials within the core of tubular electrodes that generate shielding vapors.
Figure 2 illustrates the shielding of the welding arc and molten pool with a covered “stick” electrode – the type of electrode used in most manual arc-welding. The extruded covering on the filler metal rod, under the heat of the arc, generates a gaseous shield that prevents air from contacting the molten metal. It also supplies ingredients that react with deleterious substances on the metals, such as oxides and salts, and ties these substances up chemically in a slag that, being lighter than the weld metal, arises to the top of the pool, and crusts over the newly solidified metal. This slag, even after solidification, has a protective function; it minimizes contact of the very hot solidified metal with air until the temperature lowers to a point where reaction of the metal with air is lessened.
Designing for Arc Welding
Design of Welded Joints
The loads in a welded steel design are transferred from one member to another through welds placed in weld joints. Both the type of joint and the type of weld are specified by the designer.
Figures 3 and 4 show the joint and weld types. Specifying a joint does not by itself describe the type of weld to be used. Thus, ten types of welds are shown for making a butt joint. Although all but two welds are shown with butt joints here, some may be used with other type of joints. Thus a single bevel weld may also be used in a T or a corner joint, and a single-V weld may be used in a corner, T, or butt joint.
Fillet-Welded Joints
The fillet weld, requiring no groove preparation, is one of the most commonly used welds in machine design. Various corner welds are shown in Figure 5.
The corner-to-corner joint, as in (A) is difficult to assemble because neither plate can be supported by the other. A small electrode with low welding current must be used so that the first welding pass does not burn through. The joint requires a large amount of metal.
The corner joint shown in (B) is easy to assemble, does not burn through, and requires just half the amount of weld metal as the joint in (A). However, by using half the weld size, but placing two welds, one outside and the other inside, as in (C), it is possible to obtain the same total throat as with the first weld. Only half the weld metal need be used.
With thick plates, a partial penetration groove joint, as in (D), is often used. This requires beveling. For a deeper joint, a J preparation, as in (E), may be used in preference to a bevel.
The fillet weld in (E) is out of sight and makes a neat and economical corner.
The size of welds should always be designed with reference to the size of the thinner member. The joint cannot be made any stronger by using the thicker member for the weld size, and much more weld metal will be required.
Groove and Fillet Combination
A combination of partial penetration groove weld and a fillet weld is used for many joints. The fillet welds are easy to apply and require no special plate preparation. They can be made using large-diameter electrodes with high welding currents, and as a consequence, the deposition rate is high. The cost of welds increases as the square of the leg size.
In comparison, the double bevel groove weld has about one-half the weld area of the fillet welds. However, it requires extra preparation and the use of the smaller diameter electrodes with lower welding currents to place the initial pass without burning through. As plate thickness increases, this initial low deposition region becomes a less important factor, and the higher cost factor decreases in significance.
Groove Joints
Figure 6 indicates that the root opening is the separation between the members to be joined. A root opening is used for electrode accessibility to the base or root of the joint. The smaller the angle of the bevel, the larger the root opening must be to get a good fusion at the root. If the root opening is too small, the root fusion is more difficult to obtain, and smaller electrodes must be used, thus slowing down the welding process. If the root opening is too large, the weld quality does not suffer, but more weld metal is required; this increases the welding cost and will tend to increase distortion.
Using a double groove joint in preference to a single groove cuts the amount of welding in half. This reduces distortion and makes possible alternating the weld passes on each side of the joint, again reducing distortion. Backup strips are used on larger root openings and are common when all welding must be done from one side, or when the root opening is excessive. Occasionally, the backup strips are left in place and become an integral part of the joint.
Backup strip material should conform to the base metal. Short intermittent tack welds should be used to hold the backup strip in place, and these should preferably be staggered to reduce any initial restraint of the joint. The backup strip should be in intimate contact with both plate edges to avoid trapped slag at the root.
On a butt joint, nominal weld reinforcement (approximately 1/16” above flush) is all that is necessary as shown in the figure. Additional build-up serves no useful purpose and will increase the weld cost. Care should be taken to keep both the width and the height of the reinforcement to a minimum. See Figure 7.
Codes and Specifications
Public safety is involved in the design and fabrication of pipelines and pressure vessels, and to minimize the danger of catastrophic failure or even premature failure, documents are established to regulate their design and construction. These documents are called specifications, codes, standards, and rules. Sometimes the terms are used interchangeably.
Codes and specifications are generally written by industrial groups, trade or professional organizations, or government bureaus, and each code or specification deals with applications pertaining specifically to the interest of the authoring body. Large manufacturing organizations may prepare their own specifications to meet their specific needs. Among the major national organizations that write codes that involve arc welding are the following:
- American Welding Society (AWS)
- American Institute of Steel Construction (AISC)
- American Society for Testing Materials (ASTM)
- American Society of Mechanical Engineers (ASME)
- American Petroleum Institute (API)
The construction of welded boilers and pressure vessels is covered by codes and specifications that describe, among other items, the permissible materials, size, configuration, service limitations, fabrication, heat treatment, inspection and testing requirements. These codes also outline the requirements for qualification of welding procedures and operators. Numerous state, city and local government agencies also issue codes governing pressure vessels. Commonly applied codes are:
- ASME Boiler and Pressure Vessel Code
- API Codes • General specifications for building naval vessels
- Marine engineering regulations and material specifications
- ABS rules for building and classing steel vessels
- TEMA standards
- Lloyds rules and regulations
Consumables and Machinery
Electrodes
Arc welding electrodes are identified using the A.W.S, (American Welding Society) numbering system and are made in sizes from 1/16” to 5/16”. An example would be a welding rod identified as a 1/8" E6011 electrode. The electrode is 1/8" in diameter. The "E" stands for arc welding electrode. Next will be either a 4 or 5 digit number stamped on the electrode. The first two numbers of a 4 digit number and the first 3 digits of a 5 digit number indicate the minimum tensile strength (in thousands of pounds per square inch) of the weld that the rod will produce, stress relieved. Examples would be as follows:
E60xx would have a tensile strength of 60,000 psi E110XX would be 110,000 psi. The next to last digit indicates the position the electrode can be used in.
- EXX1X is for use in all positions
- EXX2X is for use in flat and horizontal positions
- EXX3X is for flat welding
The last two digits together, indicate the type of coating on the electrode and the welding current the electrode can be used with. Such as DC straight, (DC -) DC reverse (DC+) or A.C. The thicker the material to be welded, the higher the current needed and the larger the electrode needed.
Welding Equipment
Since there are several major arc-welding processes and various stages of mechanization in each, the welding equipment, in addition to the power source, involves numerous mechanism and devices to facilitate laying a weld bead. In Figure 1, the basic welding circuit is illustrated. In this circuit is the power source, with cables running from it in one direction to the work and in the other direction to the electrode, from the tip of which the arc is struck. On each side of the power source – extending to the work or to the arc-delivering electrode – are other items of equipment needed to accomplish welding. This equipment will vary according to the welding process and its degree of mechanization. The major equipment are:
- Welding Cable
- Electrode Holder
- Ground Connections
- Semi-Automatic Gun and Wire Feeder
- Mechanized Travel Units
- Full-Automatic Welding Heads
- Equipment for Arc Heating
- Protective Equipment
Submerged-Arc Process
Submerged Arc Welding (SAW) is a high-productivity automatic welding method in which the arc is struck beneath a covering layer of flux. This increases arc quality, since contaminants in the atmosphere are blocked by the flux. The slag that forms on the weld generally comes off by itself and, combined with the use of a continuous wire feed, the weld deposition rate is high. Working conditions are much improved over other arc welding processes since the flux hides the arc and no smoke is produced. The process is commonly used in industry, especially for large products. As the arc is not visible, it requires full automation. In-position welding is not possible with SAW.
Gas Metal Arc-Welding Process
Gas Metal Arc Welding (GMAW) is a semi-automatic or automatic welding process that uses a continuous wire feed as an electrode and an inert or semi-inert shielding gas to protect the weld from contamination. When using an inert gas as shield it is known as Metal Inert Gas (MIG) welding. A constant voltage, direct current power source is most commonly used with GMAW, but constant current systems as well as alternating current can be used. GMAW welding speeds are relatively high due to the automatically fed continuous electrode, but is less versatile because it requires more equipment than the simpler SMAW process. Originally developed for welding aluminum and other non-ferrous materials in the 1940s, GMAW was soon applied to steels because it allowed for lower welding time compared to other welding processes. Today, GMAW is commonly used in industries such as the automobile industry, where it is preferred for its versatility and speed. Because it employs a shielding gas, however, it is rarely used outdoors or in areas of air volatility.
Figure 10: Gas Metal Arc Welding (GMAW)
Self-Shielded Flux Cored Process
A related process to GMAW, Flux Cored Arc Welding (FCAW), uses similar equipment but uses wire consisting of a steel electrode tube surrounding a powder fill material. This cored wire is more expensive than the standard solid wire and generates extra shielding gas and/or slag, but it permits higher welding speed and greater metal penetration.
Figure 11: Flux-Cored Welding
Gas Tungsten Arc-Welding Processes
Gas Tunsten Arc Welding (GTAW), or tungsten inert gas (TIG) welding, is a manual welding process that uses a non-consumable electrode made of tunsten, an inert or semi-inert gas mixture, and a separate filler material. Especially useful for welding thin materials, this method is characterized by a stable arc and high quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds. It can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals. It is often used when quality welds are extremely important, such as in bicycle, aircraft and naval applications. A related process, plasma arc welding, also uses a tungsten electrode but uses plasma gas to make the arc. The arc is more concentrated than the GTAW arc, making transverse control more critical and thus generally restricting the technique to a mechanized process. Because of its stable current, the method can be used on a wider range of material thicknesses than can the GTAW process and is much faster. It can be applied to all of the same materials as GTAW except magnesium; automated welding of stainless steel is one important application of the process. A variation of the process is plasma cutting, an efficient steel cutting process.
Quality Control
Weld Quality
In production welding, the term “weld quality” is relative. Generally, any weld is a good weld if it meets appearance requirements and will continue indefinitely to do the job for which it was intended.
The first step in assuring a weld quality is to ascertain the degree required by the application. A standard should be established based on service needs. Engineering performance will be the main consideration in arriving at the standard, but appearance may also be important. A safety factor must, of necessity, be built into the standard, but it should be reasonable. Once the standard has been set, it is the responsibility of everyone concerned with the job to see that it is followed.
On the low side, the predetermined standard of quality should never be compromised. On the high side, there is no objection to extra quality, provided it has been obtained at no penalty in cost. If tests repeatedly show that the welds are exhibiting a degree of quality far greater than required by the standard, a cost reduction through modification of weldment design or procedures is possible.
Frequently, the standards are preset by prevailing specifications or engineering and legal codes. Sometimes, such standards are ultra-conservative, but when they apply they must be honored. The engineer can do his company or the customer a service by pointing out unrealistic specifications and the opportunities for cost savings, but the specifications must be adhered to rigidly until revised.
Five P’s that assure quality
By giving attention to five “P’s”, weld quality will come about almost automatically, reducing subsequent inspection to a routing checking and policing activities. The five P’s are:
- Process Selection – the process must be right for the job
- Preparation – the joint configuration must be right and compatible with the welding process
- Procedures – to assure uniform results, the procedure must be spelled out in detail and following rigidly during welding
- Pretesting – by full scale mockups or simulated specimens, the process and procedures are proved to give the desired standard of quality
- Personnel – qualified people must be assigned to the job
Inspection and Testing
Visual Inspection During the Work
Visual inspection should begin before the first arc is struck. The material should be examined to see if they meet the specifications for quality, type, size, cleanliness and freedom from defects. Foreign matter that could be detrimental to the weld should be removed. The pieces to be joined should be checked for straightness, flatness and dimensions. Alignment and fit-up of parts and fixturing should be scrutinized. Joint preparation should be checked. Inspection prior to welding should also include verification that correct process and procedures are being employed.
Assuming the preliminary requirements are in good order, the most productive inspection will take place while the weldment is being fabricated. Examination of a weld bead and the end crater may reveal quality deficiencies such as cracks, inadequate penetration, and gas and slag inclusions to a competent inspector. On simple welds, inspection of a specimen at the beginning of the operation and periodically as the work progresses may be adequate. When more than one layer of filler metal is deposited, however, it may be desirable to inspect each layer before a subsequent layer is placed.
Visual Inspection after Welding
Visual inspection after the weldment has been completed is also useful in evaluating quality, even if radiographic, ultrasonic, or other methods are to be employed. Here surface flaws, such as cracks, porosity and unfilled craters can be detected, and may be of such consequence that repairs are required or the work is rejected without use of subsequent inspection procedures.
Dimensional variations from tolerances, warpage, and faults in appearance are detected visually at this stage. The extent and the continuity of the weld, its size and the length of segments in intermittent welds can be readily measured or noted.
Welds must be cleaned of slag to make inspection for surface flaws possible. A glass with a magnification of up to 10 diameters is helpful in detecting fine cracks and other defects. shot blasting should not be used in preparing the weld for examination, since the peening action may seal fine cracks and make them invisible.
Other inspection methods are briefly described below:
ULTRASONIC INSPECTION
This may be manual or mechanized. A pulse of electrical energy is fed to the probe in which a piezo-electric crystal converts it to mechanical vibrations at an ultrasonic frequency. The vibrations are transmitted (via a layer of grease to exclude the air) through the work. If they encounter a defect some are reflected back to the probe, where they regenerate an electrical signal. A cathode ray tube trace, started when the original signal is sent, displays the reflected defect signal and from it time - indicating distance from probe, and amplitude – indicating defect size, can be calculated.
Ultrasonic inspection may be used on most metal except those with coarse or varying grain structure. Typical welding applications are for welds in thick wall vessels, and welds with access to one side only.
Advantages of Ultrasonic inspection are:
- Immediate presentation of results.
- Not necessary to evacuate personnel.
- Can be battery powered.
- Depth location of defects.
The limitations of Ultrasonic inspection are:
- Trained and skilled operator needed.
- No pictorial record.
MAGNETIC PARTICLE INSPECTION
This may be manual or mechanized. The work is magnetized either by passing a current through it, or through a coil surrounding it. Defects on or near the surface disrupt the magnetic field (unless they are parallel to it). A magnetic particle fluid suspension is applied which concentrates around the defects. The work is viewed either directly or by ultra-violet light using a dye which fluoresces - i.e. emits visible light (this must be done where normal lighting is subdued). After testing, work may be demagnetized if required.
Magnetic particle inspection may be used on magnetic materials only – that includes ferritic steels and some nickel alloys. Typical welding applications are rapid inspection of welded structural details, and production rate inspection of small components.
Advantages of Magnetic particle inspection are:
- Direct indication of defect location.
- Initial inspection by unskilled labor.
- Some indication of sub-surface defects but of low sensitivity.
- Not critically dependent on surface condition.
Limitations of Magnetic particle inspection are:
- No use for non-magnetic materials.
- Defect detection critically dependent on alignment across magnetic field.
- Sub-surface flaws require special procedures.
GAMMA RADIOGRAPHY
Gamma rays, similar to X-rays but of shorter wavelength, are emitted continuously from the isotope. It cannot be ‘switched off’ so when not in use, it is kept in a heavy storage container that absorbs radiation. They pass through the work to be inspected. Parts of the work presenting less obstruction to gamma rays, such as cavities or inclusions, allow increased exposure of the film. The film is developed to form a radiograph with cavities or inclusions indicated by darker images. Section thickness increases (such as weld) appear as less dense images.
Most weldable materials can be inspected by Gamma radiography. Typical welding applications are site inspection, and panoramic exposure for small work.
Advantages, limitations, consumables and safety as for X-ray radiography.
X-ray RADIOGRAPHY
X-rays are emitted from the tube and pass through the work to be inspected. Parts of the work presenting less obstruction to X-rays, such as cavities or inclusions, allow increased exposure of the film. The film is developed to form a radiograph with cavities or inclusions indicated by darker images. Section thickness increases (such as weld under-bead) appear as less dense images.
Most weldable materials may be inspected. Typical welding applications are pipelines and pressure vessels.
Advantages of X-ray radiography are:
- Accurate pictorial presentation of results.
- Radiographs may be kept as a permanent record.
- Not confined to welds.
Limitations of X-ray radiography are:
- Personnel must be clear of area during exposure.
- Cracks parallel to film may not show up.
- Film is expensive.
DYE PENETRANT INSPECTION
This may be manual or mechanized. A special dye is applied to the surface of the article to be tested. A suitable time interval allows it to soak into any surface defects. The surface is then freed from surplus dye and the dye in the crack revealed by either: applying a white powder developer into which the dye is absorbed producing a color indication, or, illuminating with ultra-violet light under which the dye fluoresces, that is, emits visible light. This must be done where normal lighting is subdued.
Dye-penetrant inspection may be done on any - non porous material. Typical welding applications are root runs in pipe butt welds, and leak paths in containers.
Advantages of dye penetrant inspection are:
- Low cost.
- Direct indication of defect location.
- Initial examination by unskilled labor.
Limitations of Dye-penetrant inspection are:
- Surface defects only detected.
- Defects cannot readily be rewelded due to trapped dye.
- Rough welds produce spurious indications.
Sources:
1. The Procedure Handbook of Arc Welding - Lincoln Electric Company
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